Spectrally selective zinc oxide particles and methods of making thereof

ABSTRACT

Disclosed are methods of forming a method for forming spectrally selective nanoparticles, the method comprising: heating a growth solution comprising a zinc salt precursor, zinc oxide seed particles, and one or more dopants in a non-pressurized hydrothermal reactor to a first temperature under agitative conditions for a reaction period; cooling the reactor to a second temperature less than the first temperature for a cooling period to form a precipitate of recrystallized doped zinc oxide nanoparticles dispersed in a suspension; and separating and collecting the recrystallized nanoparticles from the suspension, wherein the collected nanoparticles exhibit a spectral selectivity in the atmospheric window. 
     Also disclosed herein are comprising a population of polycrystalline zinc oxide nanoparticles doped with one or more dopants, wherein the population of nanoparticles is spectrally selective in the atmospheric window.

RELATED APPLICATION

This U.S. non-provisional application claims priority to, and thebenefit of, U.S. Provisional Patent Application No. 63/365,058, filedMay 20, 2022, which is incorporated by reference herein in its entirety.

BACKGROUND

In recent decades, the growing concentration of carbon dioxide in theair has increased the atmospheric retention of thermal energy. Thiswarming effect is especially felt in urban areas, where modern urbanareas with populations of more than 1 million people are termed “heatislands” by the US Environmental Protection Agency (EPA) because theannual mean air temperature can be 1-3° C. hotter than surrounding ruralareas. Heat islands increase peak cooling loads, air conditioning costs,air pollution, greenhouse gas emissions, heat-related illness/mortality,and even water pollution. In developed countries, over 50% of urbansurface areas are either roofs or paved surfaces.

Surfaces exposed to incoming solar irradiance are prime locations forday and night passive radiative cooling technology. Radiative coolerspassively cool terrestrial objects by selectively emitting heat throughthe earth's atmospheric window to the cold of outer space whilereflecting electromagnetic radiation outside the atmospheric window.This thermal management technique can passively cool a structure withoutthe need for energy-invasive cooling devices (e.g., air conditioners),thereby reducing the demand for external energy sources. For example,Catalanotti et al. (1975) demonstrated that a 12.5 μm thick TEDLAR®(polyvinyl-fluoride plastic) film on top of an aluminum substratethrough passive radiative cooling could drop surface temperatures 12° C.below ambient temperature at night, reducing the “heat island” effect.

More recently, several commercial products have emerged as potentialradiative coolers. However, many of these radiative cooling products arespectrally indiscriminate, thereby limiting their effectiveness forradiative cooling during the daytime. Furthermore, these cooling methodsoften require expensive nanoscale precision instrumentation for uniformmulti-layer thin films or nanostructures in a controlled environment,which is currently not feasible for large-scale applications.

SUMMARY

Disclosed herein are compounds, compositions, methods for making andusing such compounds, and compositions.

The exemplary methods provide a low-cost, scalable, and tunable processfor forming spectrally selective doped zinc oxide nanoparticles. Theprocess can operate under low-temperature and low-pressure conditions,allowing for an economical synthesis of doped zinc oxide nanoparticles.

For example, the doped zinc oxide nanoparticles can be tuned for highemissivity within the atmospheric window to offer greater cooling powercompared to conventional radiative cooling technologies. Doped zincoxide nanocrystals according to the present disclosure can be utilizedin a variety of applications, such as materials for passive radiativecooling, Thermal Control Coating (TCC), and photovoltaic devices. Thepresently disclosed compositions can further exhibit improved durabilityover existing technology.

In various aspects, disclosed herein are methods of forming spectrallyselective nanoparticles, the method comprising: heating a growthsolution comprising a zinc salt precursor, zinc oxide seed particles,and one or more dopants in a non-pressurized (e.g., 2 atm or less)hydrothermal reactor to a first temperature under agitative conditionsfor a reaction period; cooling the reactor to a second temperature lessthan the first temperature for a cooling period to form a precipitate ofrecrystallized doped zinc oxide nanoparticles dispersed in a suspension;and separating and collecting the recrystallized nanoparticles from thesuspension, wherein the collected nanoparticles (e.g., nanocrystals)exhibit a spectral selectivity in the atmospheric window (e.g., from 8μm to 13 μm).

In some aspects, the zinc salt precursor comprises one or more zincsalts. In various examples, the one or more zinc salts include zincacetate, zinc nitrate hexahydrate, or a combination thereof.

In various aspects, the growth solution further comprises a surfactant(e.g., hexamethylenetetramine (HMTA), sodium dodecyl sulfate (SDS),and/or sodium lauryl sulfate (SLS)).

In some aspects, the one or more dopants is chosen from Ag₂O, Al₂O₃,CaCO₃, Cu₂O, MgO, SiC, SiO, SiO₂, SiO₄, Si₃N₄, SnO₂, TiO₂, Fe₃O₄, VO₂,SnO, CeO, and combinations thereof. In various examples, the growthsolution includes two or more dopants. In some aspects, at least one ofthe two or more dopants comprises a noble metal. In some aspects, thetwo or more dopants comprise Si₃N₄ and SiO₂. In some aspects, the zincoxide seed particles comprise zinc oxide powder. In some aspects, thezinc oxide seed particles comprise doped zinc oxide nanoparticles. Insome aspects, the zinc oxide powder has an average particle size of from25 nm to 200 nm (e.g., from 50 nm to 100 nm).

In some aspects, the nanoparticles are substantially polydisperse. Insome aspects, the nanoparticles are substantially polymorphic and/orsubstantially polycrystalline. In some aspects, the first temperatureand the second temperature are both 200° C. or less (e.g., 190° C. orless, 180° C. or less, 170° C. or less, 160° C. or less, 150° C. orless, 140° C. or less, 130° C. or less, 120° C. or less, 110° C. orless, 100° C. or less, 90° C. or less, 80° C. or less, or 70° C. orless).

In some aspects, the step of separating and collecting the doped zincoxide nanoparticles comprises decanting the solution, washing theprecipitated doped zinc oxide nanoparticles at least once (e.g., usingdeionized (DI) water), and substantially drying the doped zinc oxidenanoparticles.

In some aspects, the agitative conditions comprise continuous mechanicalstirring. For example, in various aspects, the zinc oxide seed particlesare continually suspended in the growth solution for substantially theentire reaction period.

In another aspect, the present disclosure relates to a compositioncomprising: a population of polycrystalline zinc oxide nanoparticles(e.g., nanocrystals) doped with one or more dopants, wherein thepopulation of nanoparticles is spectrally selective in the atmosphericwindow (e.g., from 8 μm to 13 μm).

In some aspects, the one or more dopants is selected from the groupconsisting of Ag₂O, Al₂O₃, CaCO₃, Cu₂O, MgO, SiC, SiO, SiO₂, SiO₄,Si₃N₄, SnO₂, TiO₂, Fe₃O₄, VO₂, SnO, CeO, and combinations thereof.

In some aspects, the one or more dopants comprise two or more dopants.For example, various aspects include wherein at least one of the two ormore dopants comprises a noble metal (e.g., a noble metal oxide and/or anoble metal nitride). In some examples, the two or more dopants includeSi₃N₄ and SiO₂.

In some examples, the composition comprising the population ofnanoparticles is included in a cooling coating (e.g., a thermal controlcoating (TCC)) to assist with heat dissipation.

In another aspect, the present disclosure relates to a nanocompositecomprising: a substrate comprising one or more layers deposited thereon,wherein at least one of the one or more layers comprises a radiativecooling layer comprising doped zinc oxide nanoparticles (e.g.,nanocrystals) are spectrally selective in the atmospheric window (e.g.,from 8 μm to 13 μm).

In various aspects, the substrate comprises aluminum, silver, an alloythereof, or a combination thereof.

In various aspects, the one or more dopants is selected from the groupconsisting of Ag₂O, Al₂O₃, CaCO₃, Cu₂O, MgO, SiC, SiO, SiO₂, SiO₄,Si₃N₄, SnO₂, TiO₂, Fe₃O₄, VO₂, SnO, CeO, and combinations thereof.

In various aspects, the doped zinc oxide nanoparticles are coupled to asurface of the substrate (e.g., using aerosol spraying and/or chemicalsolution deposition).

In various aspects, the doped zinc oxide nanoparticles comprise a noblemetal dopant configured to provide an enhanced plasmonic effect.

In various aspects, the nanoparticles are substantially polydisperse.

In various aspects, the nanoparticles are substantially polymorphicand/or substantially polycrystalline.

In various aspects, the nanoparticles comprise a volume fraction of theradiative cooling layer of from 3% to 10%.

In various aspects, the radiative cooling layer has a thickness of from40-300 μm.

In various aspects, the radiative cooling layer comprises a filler(e.g., SiO) having the doped zinc oxide nanoparticles randomlydistributed therein.

In various aspects, the radiative cooling layer is configured to exhibita radiative cooling power of 15 W/m² or more (e.g., 20 W/m² or more, 25W/m² or more, 30 W/m² or more, 35 W/m² or more, 40 W/m² or more, 45 W/m²or more, 50 W/m² or more, 55 W/m² or more, 60 W/m² or more, 65 W/m² ormore, 70 W/m² or more, 75 W/m² or more, 80 W/m² or more, 85 W/m² ormore, 90 W/m² or more, 95 W/m² or more, 100 W/m² or more).

In various aspects, the nanocomposite further includes one or morepolymer layers. Additional advantages of the disclosed subject matterwill be set forth in part in the description that follows and theFigures, and in part will be obvious from the description, or can belearned by practice of the aspects described below. The advantagesdescribed below will be realized and attained by means of the elementsand combinations particularly pointed out in the appended claims. It isto be understood that both the foregoing general description and thefollowing detailed description are exemplary and explanatory only andare not restrictive.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying figures, which are incorporated in and constitute apart of this specification, illustrate several aspects described below.

FIG. 1 shows an exemplary diagram of a hydrothermal synthesis processsetup according to one aspect of the present disclosure.

FIG. 2 shows a flow chart of the modified hydrothermal synthesis processaccording to one aspect of the disclosure.

FIG. 3A shows a Scanning Electron Microscope (SEM) micrograph of typicalhexagonal morphologies for ZnO nanoparticles.

FIG. 3B shows a Scanning Electron Microscope (SEM) micrograph ofinnovation displaying a drastic change in morphology resulting incrystalline nanosheets for doped ZnO crystals.

FIG. 3C shows a FTIR analysis for electromagnetic radiation absorptionin the infrared spectrum; inside the box is the primary atmosphericwindow. The results show how nearly pure ZnO nanoparticles obtain moreabsorption over Zinc oxide powder while depicting how spectralselectivity in the atmospheric window (box in 8-13 μm range) wasmodified by a method according to the present disclosure. Nanocrystalsdisplaying atypical shapes for ZnO crystals doped ZnO nanocrystals (FIG.3B) have a distinctive peak within the atmospheric window whichdemonstrates favorable properties for passive radiative cooling.

FIG. 4A shows a Transmission Electron Microscopy (TEM) image ofnanocrystal showing atypical morphology for ZnO nanocrystals at 50 nmmagnification.

FIG. 4B shows a Transmission Electron Microscopy (TEM) image ofnanocrystal showing doped ZnO crystalline nanosheets dispersed in manyorientations for a polycrystalline structure at 5 nm magnification.

FIG. 4C shows a Transmission Electron Microscopy (TEM) of nanocrystalshowing doped ZnO crystalline nanosheets dispersed in many orientationsfor a polycrystalline structure at 5 nm magnification.

FIG. 5 shows a flow diagram of the recrystallization process whichstarts with mixing a homogeneous solution comprised of precursorsolution with the addition of seed particles and dopants. The seedparticles serve as nucleation sites as the nanocrystals start to growwhile suspended in continuously mixed solution. Subsequently, thegrowing particles aggregate which leads to the generation of polymorphiccrystals. The polymorphic and polycrystalline nanostructures displayedfavorable spectral selectivity for passive radiation cooling.

FIG. 6 shows a graph from particle size analysis depicting two peaksrepresenting 2 particle sizes where 47.9% of the particle size was 45.7nm, while 52.1% had a particle size of 270 nm. These results confirm theeffective use of two different seed sizes in a precursor solution toproduce two different size nanoparticles in the same batch.

FIG. 7 shows a spectral analysis depicting pure ZnO nanoparticles with adistinctive absorbance peak at 378 nm wavelength, while the synthesisprocess with doping altered the material properties of ZnO nanoparticles(RC 22 and 27) to become highly reflective over the entire ultravioletand visible spectrum.

FIG. 8 shows a spectral analysis results reveal consistent levels ofhigh reflectivity of solar irradiance outside the atmospheric window(8-13 μm) with absorption peaks inside the atmospheric window equatingto higher levels of radiative cooling for doped ZnO nanoparticles (RC10, 35, 40, 41, 42) than pure zinc oxide powder and pure ZnOnanoparticles. The synthesis process can additionally tailor absorptionlevels within the atmospheric window to yield differing cooling levelswhich can be beneficial for building integration in colder climates toavoid increased heating load penalty.

FIG. 9 shows a spectral analysis for doped ZnO nanoparticles (RC 44 and48) demonstrating the synthesis process can produce favorable propertiesfor passive radiative cooling, high reflectivity of solar irradianceoutside the atmospheric window (8-13 μm) with high absorption peaks(70-85%) inside the atmospheric window.

FIG. 10 shows that the presently disclosed synthesis process and dopingof parent material ZnO (Commercial) produced changes in XRD spectra,which is indicative of modifying the parent structure.

FIG. 11 shows a spectral analysis of pure ZnO (Commercial) and doped ZnOnanoparticles (RC 41, 42, 46) demonstrates how changes in the parentstructure, such as crystallite size and lattice strain contribute toobtaining properties favorable for passive radiative cooling.

FIG. 12 shows an image of plates (top) and thermal imagery (bottom) withcolor temperature scale ambient temperature of 29° C. The lighter colorsin thermal image are the hottest, while darker colors are the coolest.(a) Cool roof coating—440 μm thick paste layer, (b) Plate 21—47 μm thickw/5% RC 37; Spray w/siloxane mixture, (c) Plate 11—40 μm thick w/5% RC39; Spray w/siloxane mixture, (d) Plate 24—58 μm thick w/5% RC 10; CSDw/siloxane mixture, (e) Plate 33—30 μm thick w/5% RC 44; Spray acrylicadhere particles.

FIG. 13 shows infrared absorption spectrum for doped ZnO nanoparticlesfrom RC 33 batch (top left) SEM image (top right); RC 42 batchdemonstrates spectral selectivity inside atmospheric window 8-13 μm(bottom left) and TEM of coated RC 33 seed particle in RC 42 batch(bottom right).

FIGS. 14A-14D show SEM micrographs of hydrothermally synthesized ZnOnanoparticles. (FIG. 14A) Spherical doped ZnO (FIG. 14B) Hexagonal pureZnO (FIG. 14C) Nanorods doped ZnO RC 33 (FIG. 14D) Multiple size dopedZnO RC 32.

FIGS. 15A-15E show SEM image of doped ZnO batches with hierarchicalsized nanoparticles. (FIG. 15A) RC 41 nanoneedle and nanosheet doubledopant ZnO (FIG. 15B) RC 33 nanorods doped ZnO (FIG. 15C) RC 36 multiplemorphology and size doped ZnO (FIG. 15D) RC 40 nanosheets with a lot ofagglomeration (FIG. 15E) RC 37 multiple size doped ZnO.

FIGS. 16A-16E show schematics of thick film nanocomposites over aluminumplate substrate (not to scale) application methods: (FIG. 16A) One-stepink layer (FIG. 16B) Two-step ink layer (FIG. 16C) Spectrally selectiveparticles adhering to binder coating on the substrate surface (FIG. 16D)One-step paste layer (FIG. 16E) Multi-layer.

FIG. 17 shows UV-Vis spectrum of aluminum substrates with nanocompositefilm coatings.

FIGS. 18A-18D show SEM micrographs of thick film nanocomposites. (FIG.18A) Surface morphology of CSD of solvent and nanoparticle mixture withspray coated binding layer (FIG. 18B) Surface of 47 μm thick filmnanocomposite ink layer shows random dispersion of spectrally selectivenanoparticles in siloxane binder (FIG. 18C) Magnification of thick filmnanocomposite ink layer's spectrally selective nanoparticles, size range4—6 μm (FIG. 18D) Surface morphology of paste layer with SiO filler anddoped ZnO nanoparticles, size range 4—8 μm (T18) in 65 μm thick film.

DETAILED DESCRIPTION

The materials, compounds, compositions, articles, and methods describedherein may be understood more readily by reference to the followingdetailed description of specific aspects of the disclosed subject matterand the Examples and Figures included therein.

Before the present materials, compounds, compositions, and methods aredisclosed and described, it is to be understood that the aspectsdescribed below are not limited to specific synthetic methods orspecific reagents, as such may, of course, vary. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular aspects only and is not intended to be limiting.

Also, throughout this specification, various publications arereferenced. The disclosures of these publications in their entiretiesare hereby incorporated by reference into this application in order tomore fully describe the state of the art to which the disclosed matterpertains. The references disclosed are also individually andspecifically incorporated by reference herein for the material containedin them that is discussed in the sentence in which the reference isrelied upon.

General Definitions

In this specification and in the claims that follow, reference will bemade to a number of terms, which shall be defined to have the followingmeanings:

Throughout the description and claims of this specification the word“comprise” and other forms of the word, such as “comprising” and“comprises,” means including but not limited to, and is not intended toexclude, for example, other additives, components, integers, or steps.

As used in the description and the appended claims, the singular forms“a,” “an,” and “the” include plural referents unless the context clearlydictates otherwise. Thus, for example, reference to “a composition”includes mixtures of two or more such compositions, reference to “thecompound” includes mixtures of two or more such compounds, reference to“an agent” includes mixture of two or more such agents, and the like.

“Optional” or “optionally” means that the subsequently described eventor circumstance can or cannot occur, and that the description includesinstances where the event or circumstance occurs and instances where itdoes not.

It is understood that throughout this specification the identifiers“first” and “second” are used solely to aid in distinguishing thevarious components and steps of the disclosed subject matter. Theidentifiers “first” and “second” are not intended to imply anyparticular order, amount, preference, or importance to the components orsteps modified by these terms.

As used herein, the term “nanoparticle” refers to particles having andiameter ranging from a lower limit of 0.5 nm, 1 nm, 5 nm, 10 nm, 25 nm,50 nm, 100 nm, or 250 nm to an upper limit of 500 nm, 400 nm, 300 nm,250 nm, 100 nm, 50 nm, 25 nm, or 10 nm, wherein the diameter may rangefrom any lower limit to any upper limit to the extent that the selectedrange encompasses any subset between the upper and lower limits. Itshould be understood that the term “nanoparticle” includes all knownshapes including, but not limited to, a sphere; a rod with a high to lowaspect ratio; a wire; a star, a tetrapod or any other multi-leggedshape; and a substantially spherical shape which may include an ovoid ora rice shape. Specific reference to “polymorphic nanoparticles” refersto a population of nanoparticles comprising a number of differentshapes.

As used herein, the term “crystalline” means any material comprising acrystal structure, including, for example, single crystal andpolycrystalline semiconducting materials. The term “polycrystalline”refers to a material comprising a plurality of grains or crystals of thematerial, which grains are bonded directly together by inter-granularbonds. The crystal structures of the individual grains of the materialmay be randomly oriented in space within the polycrystalline material.

The term “emissivity” as used herein refers to the ratio of a radiantquantity from a black body to a radiant quantity from a sample at thesame temperature.

As used herein, the term “spectrally selective” refers to a materialthat exhibits a larger average absorptivity/emissivity over a particularrange of wavelengths compared to wavelengths outside the range. Solarradiation incident upon a terrestrial surface may be partially absorbed,reflected, or transmitted. According to the First Law of Thermodynamicsthe sum of the fractional components of absorption (a), reflection (p),and transmission (τ) equal to one [32].

a+ρ+τ=1

Spectrally selective materials modify the absorption, reflection, andtransmission of electromagnetic radiation upon its surface. Passiveradiative cooling from emission through the atmospheric window requiresselective absorption. Kirchhoff's law states that spectral absorptivityand spectral emissivity of an object in thermal equilibrium are equal,for every wavelength and direction. The function for a body with aspectral and angular emissivity ε(λ, θ) of radiation has theabsorptivity (a) of a body equal its emissivity (c) at every wavelength(λ) [32]:

ελ(λ,θ)=αλ(λ,θ)

For example, a “spectrally selective material” can exhibit an averageabsorptivity/emissivity of from 0.05 or more (e.g., 0.10 or more, 0.15or more, 0.20 or more, 0.25 or more, 0.30 or more, 0.35 or more, 0.40 ormore, 0.45 or more, 0.50 or more, 0.55 or more, 0.60 or more, 0.65 ormore, 0.70 or more, 0.75 or more, 0.80 or more, 0.85 or more, or 0.90 ormore) over a particular range of wavelengths (e.g., the atmosphericwindow or from 8-13 μm). In some aspects, the spectrally selectivematerial can also exhibit a large average reflectivity (i.e., lowabsorptivity) for wavelengths outside the particular wavelength range.For example, the spectrally selective material can also exhibit anaverage absorptivity/emissivity of 0.25 or less (e.g., 0.20 or less,0.15 or less, 0.10 or less, 0.05 or less, 0.04 or less, 0.03 or less,0.02 or less, or 0.01 or less) for wavelengths outside of the particlewavelength range.

As used herein, the term “atmospheric window” refers to the window ofelectromagnetic waves in ranges of 0.7-2.5, 3-5, and 8-14 μm, whichexhibit high transmittance through the atmosphere. Within theatmospheric window, the primary atmospheric window includes the 8-13 μmwavelength range which can provide a high degree of transmittance toenhance radiative cooling effect. In some aspects, multiple absorptionmechanisms having high absorbance in these windows can be used toincrease the radiative cooling results. Whereas the maximum reflectivityof electromagnetic waves in the wavelength ranges of 0.3-0.7, 2.5-3,5-8, and 15-25 μm will also benefit radiative cooling.

As used herein, the term “dopant” refers to a material, which can beadded to a host material (e.g., a nanoparticle), that changes theelectronic characteristic(s) or the targeted wavelength(s) of radiationemission, reception, or filtering of the layer compared to theelectronic characteristic(s) or the wavelength(s) of radiation emission,reception, or filtering of the host material in the absence of suchdopant material.

Reference will now be made in detail to specific aspects of thedisclosed materials, compounds, compositions, articles, and methods,examples of which are illustrated in the accompanying Examples andFigures.

Systems and Methods

Disclosed herein are systems and methods for forming spectrallyselective zinc oxide nanoparticles. In one aspect, the method includes:heating a growth solution comprising a zinc salt precursor, zinc oxideseed particles, and one or more dopants in a non-pressurized (e.g., 2atm or less) hydrothermal reactor to a first temperature under agitativeconditions for a reaction period; cooling the reactor to a secondtemperature less than the first temperature for a cooling period to forma precipitate of recrystallized doped zinc oxide nanoparticles dispersedin a suspension; and separating and collecting the recrystallizednanoparticles from the suspension, wherein the collected nanoparticles(e.g., nanocrystals) exhibit a spectral selectivity in the atmosphericwindow (e.g., from 8 μm to 13 μm).

The present method advantageously utilizes a low-pressure hydrothermalreactor in the synthesis of spectrally selective zinc oxidenanoparticles. The non-pressurized reactor can, for example, operate ata pressure of 5 atm or less, such as 4 atm or less, 3 atm or less, 2 atmor less, or 1 atm or less. Unlike conventional methods using anautoclave, the low-pressure requirements of the present disclosureafford simpler scale-up, thereby making it more cost-effective.

In some aspects, the zinc salt precursor comprises one or more zincsalts. Suitable zinc salts include but are not limited to zinc chloride,zinc nitrate, zinc sulphate, and zinc acetate. In various examples, theone or more zinc salts include zinc acetate, zinc nitrate hexahydrate,or a combination thereof.

In various aspects, the growth solution further comprises a surfactant.The term “surfactant” refers to a molecule having surface activity,including wetting agents, dispersants, emulsifiers, detergents, foamingagents, etc. Non-limiting examples of suitable surfactants includehexamethylenetetramine (HMTA), sodium dodecyl sulfate (SDS), sodiumlauryl sulfate (SLS) and combinations thereof.

In some aspects, the one or more dopants is chosen from Ag₂O, Al₂O₃,CaCO₃, Cu₂O, MgO, SiC, SiO, SiO₂, SiO₄, Si₃N₄, SnO₂, TiO₂, Fe₃O₄, VO₂,SnO, CeO, and combinations thereof. In various examples, the growthsolution includes two or more dopants. In some aspects, at least one ofthe two or more dopants comprises a noble metal. Suitable noble metaldopants include, but are not limited to, platinum, iridium, rhenium,ruthenium, rhodium, palladium, silver, osmium, gold, combinationsthereof, and alloys thereof. In some aspects, the two or more dopantscomprise Si₃N₄ and SiO₂. In some aspects, the zinc oxide seed particlescomprise zinc oxide powder. In some aspects, the zinc oxide seedparticles comprise doped zinc oxide nanoparticles. In some aspects, thezinc oxide powder has an average particle size of from 25 nm to 200 nm(e.g., from 50 nm to 100 nm).

In some aspects, the nanoparticles are substantially polydisperse. Theterm “polydisperse” refers to a sample containing nanoparticles thatinclude a distribution of various particle sizes. In some aspects, thenanoparticles are substantially polymorphic and/or substantiallypolycrystalline.

In some aspects, the first temperature and the second temperature areboth 200° C. or less (e.g., 190° C. or less, 180° C. or less, 170° C. orless, 160° C. or less, 150° C. or less, 140° C. or less, 130° C. orless, 120° C. or less, 110° C. or less, 100° C. or less, 90° C. or less,80° C. or less, or 70° C. or less).

In some aspects, the step of separating and collecting the doped zincoxide nanoparticles comprises decanting the solution, washing theprecipitated doped zinc oxide nanoparticles at least once (e.g., usingdeionized water), and substantially drying the doped zinc oxidenanoparticles. Although the examples and appended Figures depict someillustrative configurations for decanting, washing, and drying, a numberof other techniques are known and can be incorporated into the presentmethod by one skilled in the art. For example, the method can includeusing vortex mixing to blend the contents of a container containingnanocrystal slurry and reaction components diluted in DI water. Thevortex mixing can separate the particles from remaining reactionbyproducts while maintaining a degree of agglomeration forrecrystallization of particles. Unexpectedly, the agglomeration ofnanoparticles can contribute to overall spectral selectivity of theresulting nanoparticles.

The term “agitative conditions” refers to conditions of the reactorwhereby power may be applied to a mixture to cause mixing of thecomponent of the mixture. Typically there is rotation motion of amechanism causing the mixing through an agitation device. The agitationdevice can, in some instances, be coupled with a pumping device thatcirculates the mixture within the reactor vessel. In some aspects, theagitative conditions comprise continuous mechanical stirring. Forexample, in various aspects, the zinc oxide seed particles arecontinually suspended in the growth solution for substantially theentire reaction period. The suspension of the seed particles can act ashigh surface area nucleation sites which can overcome the limitations ofother synthesis methods.

FIG. 1 shows an example schematic diagram of a system 100 for producingspectrally selective zinc oxide nanoparticles according to one aspect ofthe present disclosure. In the example scheme, the system 100 includes aheated bath circulator 102 and a thermoregulator 104 having temperaturecontrols and a timer. The heated bath circulator 102 and thermoregulator104 together are configured to monitor and isothermally circulate heatedwater through a first insulated conduit 106 to the interstitial spacesurrounding a jacketed reactor vessel 108 (e.g., a beaker). The water iscontinuously returned to the heated bath circulator 102 through a secondinsulated conduit 112. The interior of the reactor vessel 108 contains aprecursor solution, surfactants, solvents, seed particles and dopants ina non-pressurized environment. The reactor vessel 108 further includes amechanical stirrer 110 having a timer and temperature gauge. In variousaspects, some systems may further utilize a fume hood or distillationapparatus to eliminate or reduce emissions and recover compounds.

The reaction vessel 108 is configured to operate under conditionseffective such that a precipitate can form. After the precipitationphase, a combination of a vortex mixer 114 and a centrifuge 116 is usedfor repeated cycles 118 until the resultant liquid above the sedimentedparticles is substantially clear. The sedimented particles are thentransferred into an oven 120 where excess liquid can be removed. The dryparticles are then removed from the oven 120 and crushed using a mortarand pestle 122 to form a powder comprising spectrally selective zincoxide nanoparticles. FIG. 2 further provides an exemplary flow chart ofa synthesis process for forming spectrally selective nanoparticlesaccording to one aspect of the present disclosure.

Nanoparticle Compositions

Also disclosed herein are compositions comprising: a population ofpolycrystalline zinc oxide nanoparticles (e.g., nanocrystals) doped withone or more dopants, wherein the population of nanoparticles isspectrally selective in the atmospheric window (e.g., from 8 μm to 13μm). In various aspects, the composition includes a zinc oxide which canprovide properties favorable for passive radiative cooling. By way ofnon-limiting example, wurtzite zinc-oxide has a high thermalconductivity of 50 W/mK at room temperature to deliver efficient heatremoval with a high rate of heat transfer, which is anothercomplimentary property for effective radiative cooling. Other favorableproperties are a high heat capacity (˜40.3 J·K-1 mol-1), low thermalexpansion, and high melting temperature (1,974° C.).

In some aspects, the one or more dopants is selected from the groupconsisting of Ag₂O, Al₂O₃, CaCO₃, Cu₂O, MgO, SiC, SiO, SiO₂, SiO₄,Si₃N₄, SnO₂, TiO₂, Fe₃O₄, VO₂, SnO, CeO, and combinations thereof. Insome aspects, the one or more dopants comprise two or more dopants. Forexample, various aspects include wherein at least one of the two or moredopants comprises a noble metal (e.g., a noble metal oxide and/or anoble metal nitride). Suitable noble metal dopants include, but are notlimited to, platinum, iridium, rhenium, ruthenium, rhodium, palladium,silver, osmium, gold, combinations thereof, and alloys thereof. In someexamples, the two or more dopants include Si₃N₄ and SiO₂.

In some examples, the composition comprising the population ofnanoparticles is included in a cooling coating (e.g., a thermal controlcoating (TCC)) to assist with heat dissipation.

In another aspect, the present disclosure relates to a nanocompositecomprising: a substrate comprising one or more layers deposited thereon,wherein at least one of the one or more layers comprises a radiativecooling layer comprising doped zinc oxide nanoparticles (e.g.,nanocrystals) are spectrally selective in the atmospheric window (e.g.,from 8 μm to 13 μm).

The term “radiative cooling layer” used herein refers to a layer, or aplurality of layers of the nanocomposite, wherein emission ofelectromagnetic radiation in the spectrum with wavelengths in theatmospheric window is 30% or more (e.g., 35% or more, 40% or more, 45%or more, 50% or more, 55% or more, 60% or more, 65% or more, 70% ormore, 75% or more, 80% or more, 85% or more, 90% or more, 95% or more,or 99% or more). when the radiative cooling layer is exposed tosunlight. In various examples, the composition is configured to providea passive radiative cooling effect during both day and night.

In various aspects, the substrate comprises aluminum, silver, an alloythereof, or a combination thereof. In various aspects, the substratecomprises a polymer.

In various aspects, the one or more dopants is selected from the groupconsisting of Ag₂O, Al₂O₃, CaCO₃, Cu₂O, MgO, SiC, SiO, SiO₂, SiO₄,Si₃N₄, SnO₂, TiO₂, Fe₃O₄, VO₂, SnO, CeO, and combinations thereof.

In various aspects, the doped zinc oxide nanoparticles are coupled to asurface of the substrate (e.g., using aerosol spraying and/or chemicalsolution deposition).

In various aspects, the doped zinc oxide nanoparticles comprise a noblemetal dopant configured to provide an enhanced plasmonic effect.

In various aspects, the nanoparticles are substantially polydisperse.

In various aspects, the nanoparticles are substantially polymorphicand/or substantially polycrystalline.

In various aspects, the nanoparticles comprise a volume fraction of theradiative cooling layer of from 1% to 10%, such as from 2% to 10%, from3% to 10%, from 4% to 10%, from 5% to 10%, from 6% to 10%, from 7% to10%, from 1% to 9%, from 1% to 8%, from 1% to 7%, from 2% to 8%, from 3%to 7%, or from 4% to 6%. In various aspects, the radiative cooling layerhas a thickness of from 40 μm to 300 μm (e.g., from 50 μm to 300 μm,from 100 μm to 300 μm, from 100 μm to 250 μm, from 50 μm to 200 μm, orfrom 100 μm to 150 μm). In various aspects, the radiative cooling layercomprises a filler (e.g., SiO) having the doped zinc oxide nanoparticlesrandomly distributed therein. Some other suitable fillers includealuminum silicate, silicon dioxide, calcium carbonate, barium sulfate,talcum powder, titanium dioxide, zinc sulfide, ceramic powder, or acombination thereof.

In various aspects, the radiative cooling layer is configured to exhibita radiative cooling power of 15 W/m² or more (e.g., 20 W/m² or more, 25W/m² or more, 30 W/m² or more, 35 W/m² or more, 40 W/m² or more, 45 W/m²or more, 50 W/m² or more, 55 W/m² or more, 60 W/m² or more, 65 W/m² ormore, 70 W/m² or more, 75 W/m² or more, 80 W/m² or more, 85 W/m² ormore, 90 W/m² or more, 95 W/m² or more, 100 W/m² or more).

Although by itself the spectrally selective layer can provide equal orgreater cooling power levels than non-spectrally selective materialswith a thinner coating and less particles. However, performance can beincreased with additional layers. For example, in various aspects, thenanocomposite further includes a convective layer on top and reflectivelayer on bottom. The top convective layer can reject (e.g., reflects andscatters) the strong solar insolation in the 0.3-2.4 μm wavelengthrange, while being somewhat transparent to mid range infrared radiationcompliments the spectral selective layer. A bottom layer having highlevels of reflectivity across the spectrum can additionally be used tofurther increase cooling power.

In various aspects, the nanocomposite further includes one or morepolymer layers. By way of non-limiting example, the one or more polymerlayers can include: ethyl cellulose, poly ethyl methacrylate (PEMA),poly methyl methacrylate (PMMA), polyvinyl butyral (PVB), celluloseacetate, polyethylene, polypropylene, polyethylene terephthalate (PET),polyethylene naphthalate (PEN), polyesters, polycarbonates, or acombination thereof.

In some aspects, the nanocomposites are used to coat surfaces ofarticles and structures such as buildings, vehicles and the like. Somenon-limiting examples of coated surfaces include exterior surfaces ofoffice buildings, industrial buildings, residential buildings, sportsstadiums, and vehicle body panels.

EXAMPLES

To further illustrate the principles of the present disclosure, thefollowing examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how thecompositions, articles, and methods claimed herein are made andevaluated. They are intended to be purely exemplary of the invention andare not intended to limit the scope of what the inventors regard astheir disclosure. Efforts have been made to ensure accuracy with respectto numbers (e.g., amounts, temperatures, etc.); however, some errors anddeviations should be accounted for. Unless indicated otherwise,temperature is ° C. or is at ambient temperature, and pressure is at ornear atmospheric. There are numerous variations and combinations ofprocess conditions that can be used to optimize product quality andperformance. Only reasonable and routine experimentation will berequired to optimize such process conditions.

Example 1: Spectrally Selective ZnO Nanocrystals

Disclosed herein is an example of a low cost, scalable, non-pressurized,low temperature, constant stir, hydrothermal batch process to synthesizeselectively emitting doped zinc oxide (ZnO) semiconductor nanoparticlesfor passive radiative cooling. To enhance the passive radiating coolingeffect, nanoparticles should reflect most of the incident solarradiation, while also have strong absorption within the primaryatmospheric window (8-13 μm wavelength) to emit infrared radiation(heat) through the atmosphere to the cold space. Currently, one materialdoesn't possess these spectrally selective properties for radiativecooling. Since nanotechnology can alter bulk material properties,synthesis of nanoparticles exhibiting spectral selectivity in the rangeof 8-13 μm will advance and enhance state-of-the art passive coolingtechnology.

Materials

The experiment utilized safe and cost-effective materials having littleto no toxicity. In an aspect, the precursor solution included eitherzinc acetate (Zn(C₂H₃O₂)₂ or C₄H₆O₄Zn) or zinc nitrate hexahydrate(Zn(NO₃)₂·6H₂O) and hexamethylenetetramine (HMTA), (CH₂)₆N₄, also knownas methenamine or hexamine, sodium dodecyl sulfonate (SDS),CH₃(CH²)₁₁SO₄Na, sodium lauryl sulfate (SLS), CH₃(CH₂)₁₁SO₄Na and zincoxide powders <100 nm and <50 nm nominal sizes, all were purchased fromSigma Aldrich.

Experimental Setup

Presented is a low cost, scalable, non-pressurized, low temperature,constant stir, hydrothermal batch process to synthesize selectivelyemitting nanoparticles for passive radiative cooling. Thisnon-pressurized hydrothermal batch process is a safe, easily scaled, andlow energy alternative compared to autoclave synthesis and yields morenanoparticles. Other methods cannot be scaled upward as easily due tolimited size of autoclave reaction vessel or small specially preparedsubstrates in solution. For example, some limitations of autoclavesinclude high temperature/energy requirements, pressurized reactions, andlow working volumes [178]. Remarkably, experimental synthesis of purezinc oxide (ZnO) nanoparticles exhibited properties to enhance radiativecooling, yet synthesis experiments did not garner spectral selectivityuntil the particular chemical combinations, dopants, and synthesisprocess manipulations were used.

Initial experiments focused on evaluating the selectivity of differentdopants. Dopants which contributed to spectral selectivity weresubjected to repeated experiments to verify repeatability. Next,variations in temperature, dopant amounts, dopant combinations, seedsize, surfactants, and solvent type were examined for their effects onthe resulting nanoparticle products.

Experimental Procedure

The non-pressurized hydrothermal batch process utilized a ThermoScientific SAHARA Series heated bath circulator to provide a constanttemperature to 500 mL reactor beaker with a magnetic stirrer for thesynthesis of nanoparticles. To demonstrate the feasibility of low energysynthesis methods, reaction temperatures were administered in the rangeof 60-95° C.

The reactor beaker contained the mixture of precursor and seed in 500 mlof deionized water (DI) water. Either the precursor or growth solutionhad a 26 mM concentration of zinc acetate (Zn(C₂H₃O₂)₂ or C₄H₆O₄Zn) orzinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and hexamethylenetetramine(HMTA), (CH₂)₆N₄. Magnetic stirring was kept at a constant rate duringthe reaction for each dopant to ensure identical growth parameters forthe samples investigated.

The synthesis reaction terminated after ceasing heating and stirring andthe solution cooled to room temperature. After the reaction termination,the particles precipitated and settled to the bottom of the reactionbeaker. The remaining DI water above the nanoparticles was decantedusing a syringe. The resulting precipitant was then rinsed by placingthe concentrated solution in a centrifuge tube with clean DI water, thenthe tube was placed on a vortex mixer for 30 seconds prior tocentrifuging at 4700 rpm for 7 minutes. The cloudy water above thenanoparticles was removed and replaced with clean DI water. The tube wasagain placed on vortex mixer and rinsed as described above for two ormore times until remaining DI water above nanoparticles was clear ofreaction byproducts. The remaining nanoparticle concentration was ovendried at 100° C. for ˜15 hours, until all the moisture was removed. Thelayer of dried particles was then pulverized using a mortar and pestleto form dried particle batch into a powder.

Several parameters can be manipulated to effectively control the finalproperties such as particle size, morphology, chemical combinations, andcrystalline structure. For example, changing the growth solutionconcentration will modify the size of particles and temperature is onevariation to modify morphology [44]. Each nanoparticle batch isreferenced by the prefix RC (radiative cooling) followed by a batchnumber.

Nucleation Site Seeding

The experiment used a random dispersion of doped ZnO nanoparticles toform a nanocomposite material. Seeding layers in other synthesis methodstypically include a small specially prepared substrate inside a reactionvessel to promote aligned or epitaxial growth of nanoparticles on thesubstrate. Since this experiment examined the random dispersion of dopedZnO nanoparticles to form a nanocomposite material, there was no seedinglayer used during the hydrothermal synthesis. Instead, the seedparticles functioned as nucleation sites in the growth solution in areaction vessel. These seed particles were constantly suspended bymagnetic stirring which increased nanoparticle yield since thenucleation site was not limited to the dimensions of the speciallyprepared substrate. This unique nanoparticle synthesis seeding methodalso presents a cost-effective and scalable synthesis process forradiative cooling compared to conventional methods. For example, therelatively low-tech autoclave synthesis process has a limited sizeautoclave reaction vessel placed in high temperature oven whichrestricts scaling.

The seed particles provided a surface nucleation where dissolved solutecould grow into nanocrystals. This process eliminated a major limitingfactor for upward scaling found in current state of the art researchwhich rely upon small specially prepared substrates for aligned orepitaxial nanoparticle growth [42]. The nanoparticle-based radiativecooling thick film coating had randomly distributed particles in amedium. Additionally, the seed size and chemical composition used in theprocess can affect the nanoparticle size, which leads to another processmodification presented in this experiment to synthesize multiple sizeparticles in one hydrothermal reactor batch. Initial experimentsdiscovered multiple (aka hierarchical) particle sizes in a matrixexposed to electromagnetic radiation increased spectral selectivityrequired for radiative cooling. The multiple-size modification resultsfrom using nano powders of varying particle sizes to function as anucleation site (seed) in colloidal solution for nanoparticle growth.Experimental studies with ZnO nano powders with particle sizes <50 nmand <100 nm produced resultant products of different sizes.

Dopants

The addition of dopants can change how a ZnO nanoparticle interacts withthe electromagnetic spectrum. Zhou et al. (2014) optimized aluminumnitrate doping for zinc oxide nanoparticles which are commonly referredto AZO nanoparticles to tune absorbance in the 800 nm to 2500 nmwavelength range [262]. However, there is sparse research showingchemical doping of zinc oxide nanoparticles to obtain spectralselectivity for passive radiative cooling in the 8,000 nm to 13,000 nm(8-13 μm) range. Some exemplary candidates for obtaining selectivityinclude metal oxides and nitrides.

In this experiment, single dopants combined with precursor solution andsurfactant demonstrated spectral selectivity. Some of the dopantsinvestigated include Ag₂O, MgO, SiC, SiO, SiO₂, Si₃N₄, SnO₂, TiO₂,Fe₃O₄, Al₂O₃, SnO, and CeO. In some batches, a single dopant showed highspectral selectivity in the investigated range of wavelengths. Furtherexperiments were conducted combining more than one dopant. For example,batch RC 41 introduced two dopants of Si₃N₄ and SiO₂ early in theprocess—selectivity within the window consisted of double peaks higherthan single dopants however less absorptance over the whole atmosphericwindow was observed.

Surfactants

Another component of the growth solution, hexamethylenetetramine (HMTA),(CH₂)₆N₄, also functioned as a surfactant. Adding another surfactantlike sodium dodecyl sulfate (SDS) or sodium lauryl sulfate (SLS),CH₃(CH₂)₁₁SO₄Na, decreased surface tension and assisted with dispersingand suspending nanoparticles in solution to prevent agglomeration duringsynthesis [47].

Agglomeration

It was discovered that not only different sized nanoparticles enhancedradiative cooling, but also the agglomeration did not adversely affectthe spectral properties. Agglomeration was purely visible, yet thesample produced high levels of radiative cooling. Nevertheless,sonication of nanoparticle mixtures over 30 minutes greatly reducedagglomeration.

Characterization of Nanoparticles

The nanoparticles were characterized using Fourier Transform Infrared(FTIR) spectroscopy, UV-Vis spectroscopy, Scanning Electron Microscope(SEM), X-ray diffraction (XRD), Zeta sizer, and thermal testingtechniques.

Spectral analysis by Fourier Transform Infrared (FTIR) Spectrometerverified synthesis of spectrally selective nanoparticles. The infraredspectroscopy for nanoparticles was performed using the Perkin ElmerSeries 100 FTIR with Attenuated Total Reflectance (ATR) attachment.Sample preparation for ATR spectral analysis involved powder form ofnanoparticles upon diamond ATR lens.

The infrared spectrum for pure ZnO powder (<100 nm) has low infraredabsorption. When this powder was introduced into a growth solution forhydrothermal synthesis of ZnO nanoparticles the spectrum significantlychanged with absorption rapidly increasing from 6-12 μm and kept highlevels past 15 μm. Despite this increase in absorbance, these broadbandnanoparticles displayed no selectivity for wavelengths in 8-13 μm rangeof the atmospheric window.

Replication of Si₃O₄ doped ZnO hydrothermal batch synthesis of RC 10,35, and 40 produced relatively consistent results despite minorvariations in the process. Without wishing to be bound by theory, theincreased selectivity of batch RC 40 was hypothesized to be attributedto the improved rinsing process with the introduction of centrifuging toget rid of more reaction byproducts but also exerted a force onparticles to replicate the sedimentation process and further assist withrecrystallization. The spectra of RC 41 largely reflected the use of 2dopants, SiO₂ and Si₃N₄, which have different absorption peaks.Increased absorption peaks at ˜10.75 μm and at ˜11 μm exceeded the peaksfor single dopant while still maintaining spectral selectivity.

Multiple sized nanoparticle batch, RC 33, FIG. 13 , was unable to obtainclose to ideal spectral selectivity in atmospheric window for radiativecooling. However, when RC 33 nanoparticles were used as seed forprecursor solution in a later batch (RC 42), the atmospheric spectralselectivity with an absorption peak at 10.6 μm and increasedreflectivity outside the atmospheric window 8-13 μm was attained. The RC42 nanoparticles TEM image, FIG. 13 bottom right, depicts RC 33nanoparticle coated with new ZnO crystallization. Thus, the experimentunexpectedly observed that spectral selectivity could be achieved byusing doped ZnO nanoparticles as a seed in growth solution of thehydrothermal synthesis process which coated the nanoparticles with a newcrystal layer. Replication of the atmospheric spectral selectivityshowing high absorption within the atmospheric window (8-13 μm) andminimal absorption outside the window with doping ZnO nanoparticlesduring the hydrothermal synthesis process is demonstrated in FIG. 8 .

Ultraviolet and Visible Spectroscopy

Ultraviolet and visible spectroscopy (UV-Vis) spectroscopy equipmentincluded the Ocean Optics USB-2000 UV-Vis-NIR spectrometer and theDeNovix DS-11 Spectrophotometer/Fluorometer. This equipment alsoincluded the near infrared spectrum (NIR) into the analysis. UV Visspectral analysis verified certain nanoparticle doping combinationsproduce enhanced reflection and/or scattering. The pure ZnOnanoparticles labeled as ZnO 16 hr. in FIG. 7 had a distinct absorbancepeak at 378 nm wavelength, while doped ZnO nanoparticles from batch RC22 are highly reflective over the entire UV-Vis spectrum.

Nanoparticle Size Distribution

Particle size distribution measurements for each batch were conductedwith Malvern Zetasizer nano series instrument using a quartz cuvettewith nanoparticles dispersed by sonication of at least 25 minutes in DIwater. Normally with batch nanoparticle synthesis one particle sizebecomes predominant. To increase spectral selectivity favorable forpassive radiative cooling experimentation into obtaining more than onesize of nanoparticle per batch was conducted.

The RC 24 batch in FIG. 6 had two peaks graph representing 2 particlesizes where 47.9% of the particle size was 45.7 nm, while 52.1% had aparticle size of 270 nm. These results confirm the successful use of twodifferent seed sizes in precursor solution the batch to produce twodifferent size nanoparticles in the same batch. This process can befurther modified to obtain particle size combinations offering favorableresults for the particular application.

Scanning Electron Microscope (SEM)

The SEM provides visual analysis of nanoparticle size and morphology.The Hitachi 5800 and the Hitachi SU70 SEMs were utilized in this study.The hydrothermal process batch process in this experiment can produce awide array of sizes and morphologies as seen in FIGS. 14A-14D. FIGS.15A-15E shows the batches with high spectral selectivity unexpectedlycome in many shapes and sizes, which is contrary to other researchemphasizing uniform morphology and size. The resulting nanoparticlesshow a crystalline structure and exhibit strong agglomeration like thecrystalline nanosheets. Nevertheless, most of these nanoparticle batchesobtained higher spectral selectivity compared to commonly used radiativecooling compounds found in the leading commercially available cool roofcoatings and passive radiative cooling research.

Transmission Electron Microscopy (TEM)

The TECNAI F20 TEM with a point resolution of 0.24 nm (2.4 Angstrom) andmagnification range of 25×-1030 k× was used for analytical nanostructureanalysis. Transmission Electron Microscope (TEM) analysis for RC 40revealed recrystallization in the synthesis process resulting incrystalline nanosheets in multiple orientations fused together withrandom crystallographic orientations to form a polycrystalline structureas seen in FIGS. 4A-4C. The crystalline nanosheets in multipleorientations observed by the TEM analysis for RC 40 contribute to thespectral selectivity of these nanoparticles. The random arrangement andmultiple sizes of crystals in FIGS. 4A-4C clearly depict the uniquestructure which obtains favorable spectral selectivity.

X-Ray Diffraction (XRD)

The crystalline structure of doped ZnO nanoparticles was examined with aPowder X-ray diffraction (XRD) model PANalytical X'Pert Pro MRD systemwith Cu Kα radiation (wavelength=1.5442 Å) operated at 40 kV and 40 mA.XRD is an analytical technique for determining the degree ofcrystallinity of a sample as well as what crystal structures exist. Thespacing of the various peaks also gives the information for determiningthe crystal structure lattice parameters. Also, XRD can be used forphase identification of a crystalline material and can provideinformation on unit cell dimensions. These diffracted X-rays are thendetected, processed, and counted. By scanning the sample through a rangeof 20 angles, all possible diffraction directions of the lattice shouldbe attained due to the random orientation of the powdered material.Conversion of the diffraction peaks to d-spacings allows identificationof the mineral because each mineral has a set of unique d-spacings.Typically, this is achieved by comparison of d-spacings with standardreference patterns [40].

Data is presented as peak positions at 20 and X-ray counts (intensity)in the form of a table or an x-y plot. Intensity (I) is either reportedas peak height intensity, that intensity above background, or asintegrated intensity, the area under the peak. The relative intensity isrecorded as the ratio of the peak intensity to the most intense peak(relative intensity=I/I1×100). Peak positions occur where the X-ray beamhas been diffracted by the crystal lattice. The unique set of d-spacingsderived from this patter can be used to ‘fingerprint’ the mineral. Broaddiffraction peaks are typically attributed to a smaller particlesize.[48]

Scherrer Equation (Equation 1) can be used to determine crystallitesize:

$\begin{matrix}{P = \frac{k\lambda}{\beta\cos\theta}} & (1)\end{matrix}$

where P is the average crystallite size, k (k=0.9) is particle shapefactor, λ is the X-ray wavelength (0.1542 nm), β is the angular linewidth of half-maximum intensity and θ is the Bragg's angle.

UV-Vis spectroscopy equipment included the Ocean Optics USB-2000UV-Vis-NIR spectrometer and the DeNovix DS-11Spectrophotometer/Fluorometer. UV Vis spectral analysis verified certainnanoparticle doping combinations produce enhanced reflection and/orscattering. The pure ZnO nanoparticles labeled as ZnO 16 hr in FIG. 7had a distinctive absorbance peak at 378 nm wavelength, while doped ZnOnanoparticles from batch RC 22 were highly reflective over the entireUV-Vis spectrum.

Particle size distribution measurements for each batch were conductedwith Malvern Zetasizer nanoseries instrument using a quartz cuvette withnanoparticles dispersed by sonication in DI water. The RC 24 batch hadtwo peaks graph representing 2 particle sizes where 47.9% of theparticle size was 45.7 nm while 52.1% had a particle size of 270 nm.These results confirm the use of 2 different seed sizes in the batchprocess produced 2 different size nanoparticles in the same batch.

Although the hydrothermal process could produce spherically shapednanoparticles, further experimentation with spherical nanoparticles didnot resume since better passive radiative cooling results were obtainedwith other morphologies. XRD analysis verified new peaks obtained fromthe crystallographic planes of hexagonal ZnO show that the dopantcombinations altered the ZnO crystalline structure. For example, twodifferent dopants for batch RC 32 and RC 38 produced nanocrystals withdiffering peak intensities for same peak positions and extra peaks forRC 38 verify different crystal systems.

FIG. 10 illustrates a stacked comparison of XRD analysis on threenanocrystalline semiconductor batches RC 41, RC 42, and RC 46 to thecommercially available ZnO powder the bottom pattern. The crystallitesize for the ZnO phase were calculated using Scherrer's formula for thecommercial ZnO powder, RC 41, RC 42, and RC 46 were 323 nm, 269 nm, 296nm, and 340 nm respectively. Table 1 below provides a summary of howcrystallite size (aka grain size) and lattice strain of the parentstructure pure ZnO (Commercial) was modified by the synthesis process indoped ZnO nanoparticles (RC 41, 42, 46). The variations in thesemiconducting crystal parameters are likely contributing to differentspectral selectivity observed in this research. Shifting or new XRDpeaks are an indication of structural changes in a material. The dopingof the pure zinc oxide material resulted in many changes to the XRDspectra, which can partially be attributed to the dopant modifying thepure zinc oxide structure.

TABLE 1 XRD analysis of modified ZnO nanoparticles Crystallite LatticeSamples Phase Size (nm) Strain (%) Commercial ZnO 323 0.02 Sample 41 ZnO269 0.06 Sample 42 ZnO 296 0.156 Sample 46 ZnO 340 0.05

The present hydrothermal batch synthesis method for doping ZnOnanoparticles was used to obtain spectrally selectivity for radiativecooling. Spectral analysis of doped nanoparticles verified selective andenhanced absorption within the primary atmospheric window. This studyadditionally demonstrated the ability to precisely tune a spectrallyselective coating depending on the particular application parameters.Here, the nanoparticles introduced were tuned to be selectively emittingin the atmospheric window and highly reflective outside the window. Thepresent experimental results demonstrate a cost effective, safe, andscalable process mechanism for producing spectrally selectivenanoparticles.

Example 2: Spectrally Selective Thick Film Nanocomposite

A spectrally selective thick film nanocomposite was formed using thedoped zinc oxide nanoparticles from Example 1. The fabrication conceptin this research is akin to microelectronic thick film fabrication onlyin the sense that it's also an additive process which entails theapplication of thicker paste layers and thinner ink layers uponsubstrate as needed. The layers are added sequentially to the substrateto create a thick film nanocomposite with the desired properties. Theresultant performance is dependent upon many factors like substrateproperties, nanoparticle properties, medium properties, and climaticconditions. Another departure is much lower energy for fabricationprocess since high curing temperatures are not required. The binders(mediums) in the film adhere to surfaces and dry at standard ambienttemperatures to be cost effective and scalable like cool roof coatings.

Application Methods

The application methods in this research are chemical solutiondeposition (CSD) and/or spray coating of a paste and/or ink layers upona substrate in FIGS. 16A-16E. This research employs easy-to-applymediums to form a thick film containing a random distribution ofparticles. This is a departure from many passive radiative coolingfabrication and limited scale application methods requiring controlledenvironment and multiple layers with precise nanoscale thickness.

The thick film was composed of robust materials which could withstandoutdoor exposure to temperature extremes and solar radiation. Inaddition to the low-cost scalable application options layering optionsare introduced to account for differing environmental conditions andsubstrate properties. Shown in FIG. 16A-16E are five cost effectivethick film nanocomposite structures fabricated by simple applicationmethods of CSD and/or spraying. Note, the illustrations are not toscale. One-step ink layer, FIG. 16A, has aerosol spraying ofnanoparticles and medium mixture to form an ink layer. Experiments withsiloxane and xylene 50:50 mixture formed the medium for nanoparticles.The volume fraction of particles was 5%. Two-step ink layer, FIG. 16B,is deposition/spraying of a solvent and nanoparticle mixture onsubstrate. The solvent must exhibit good wettability to spreadnanoparticles uniformly over surface. Ethanol and isopropanol solventsprovided excellent wettability an even coating over substrates withoutcracking. Enables direct contact of nanoparticles with metal surface,which due to noble metal doping of semiconductor also enhanced plasmoniceffect. After solvent evaporates is to spray a binder coating over thenanoparticle layer to adhere, encapsulate, and form a protectivecovering over the substrate. Acrylic paint over dried nanoparticle layerworked not only to encapsulate and adhere nanoparticles to surface, butalso provided complimentary absorption in atmospheric window.

Two-step ink layer, FIG. 16B, is deposition/spraying of a solvent andnanoparticle mixture on substrate. The solvent must exhibit goodwettability to spread nanoparticles uniformly over surface. Ethanol andisopropanol solvents provided excellent wettability an even coating oversubstrates without cracking. Enables direct contact of nanoparticleswith metal surface, which due to noble metal doping of semiconductoralso enhanced plasmonic effect. After solvent evaporates is to spray abinder coating over the nanoparticle layer to adhere, encapsulate, andform a protective covering over the substrate. Acrylic paint over driednanoparticle layer worked not only to encapsulate and adherenanoparticles to surface, but also provided complimentary absorption inatmospheric window.

Spectrally selective particles adhering to binder coating on substratesurface in FIG. 16C. Application of binder to a surface followed byadhesion of spectrally selective particles on top of binder. The topsurface of particles not embedded in binder are exposed to theatmosphere like the grains in tar-based roofing shingles or rolls.One-step paste layer, FIG. 16D, has a higher particle to volume ratiothan the ink layer and can consist of a mixture of bulk materials,binder, fillers, and nanoparticles. Cool roof coatings are simply pastelayers which according to specification are to be of an averagethickness of 558.8 μm (22 mil) after application. Deposition methods caninclude but are not limited to self-leveling or assisted level of pastedeposited within borders of desired coating thickness, brushing,spraying, or rolling.

Multi-layer, FIG. 16E, is an additive combination of pastes and/or inklayers multiple layer process, can progress to become a solution fortuning radiative cooling power with different mixtures and layeringcombinations for different climatic conditions. This research hasdemonstrated that ink layers alone over a reflective substrate canprovide radiative cooling. Incorporating a reflective bottom layer willbe crucial to obtain daytime radiative cooling with ink layers. Inklayers can be below or above paste layer. However, this study has alsodemonstrated that the addition of an ink layer over a paste layer canincrease selectivity.

Layering Options

The environmental conditions, substrate properties, and factor into whattype of layering option may be applied to surface. The film may consistof one or more layers embedded with spectrally selective radiativecooling nanoparticles and/or fillers to enhance or tailor radiativecooling. The endless combinations this approach provides to tailor thickfilm nanocomposite coatings embedded and the layering of the for colderclimate locations to avoid a heating load during winter months whileproviding a robust contiguous coating on surfaces exposed to solarradiation. The order of these materials/structures facing the sun doesmatter. If the substrate is a back reflector like aluminum only onelayer upon the substrate will suffice if it contains spectrallyselective radiative cooling nanoparticles. However, the film must bethick enough to lessen the convective and conductive heat transfer tothe aluminum. The first layer for a non-reflective surface close to ablack body absorber should be a reflective layer, unless a highlyreflective cover or shield is a top layer The fabrication methodpresented is a cost-effective and robust departure from experimentalstudies. The equipment is rather simple and inexpensive low investment,low energy, and capable of large-scale application. The nanoscaleprecision is found in the spectrally selective nanoparticles and fillersembedded in film mediums.

Film Thickness

A thin film is not feasible for real world structural coatingapplications since it can be 1000 times thinner than a 100-micrometerthick film. Finding an optimal thickness of coating is a fabricationgoal, however, an optimal range of thicknesses is more realistic withsimpler lowercost application methods. An engineering tolerance isnecessary to provide radiative cooling for variations in film thicknessdue to different substrate properties and uneven surfaces since thethick film nanocomposite coating will have variations in thickness overlarge surface areas. So, the focus of this research includes obtainingradiative cooling with more durable thick films. Reflective substratescan radiatively cool with a thinner ink coating. Published experimentalwork found that maximum diffuse reflectance around 97% was achieved witha 0.25-mm (250 μm) thick coating having 17% pigment to binder weightratio [22]. This research found that for a reflective substrate a filmthickness of 40-300 μm can still provide radiative cooling. However,highly absorbing substrates require a thicker paste coating or areflective coating on the materials surface. This research observed alower level of cooling for films less than 400 μm in thickness whichagrees with reports that material properties become independent ofsubstrate at thickness of 400 μm or greater [22].

Volume Fraction

Random particle distribution within a medium is one of the simplest andcost-effective methods to provide daytime “radiative cooling”experimentally demonstrated in research. It is interesting to note thatlow volume fractions ˜4-5% nanoparticle concentration with a reflectivesubstrate can obtain daytime radiative cooling [19, 20, 21, 23, 24].Particle concentrations greater than 25% are predicted to achieve agreater degree of radiative cooling yet are not fully explored inresearch due to limitations in application and mediums. The thick filmin this research accommodates these higher volume fractions. Also, acommercially available cool roof coating can attain “reflective cooling”with a much higher volume fraction, between 38-79%, of random particlesdistributed in a binder [13]. The volume fraction of particles to themedium can be determined with Equation 2:

$\begin{matrix}{f_{v,\exp} = \frac{\left( {X_{p}/\rho_{x}} \right)}{\left( {X_{p}/\rho_{x}} \right) + V_{y}}} & (2)\end{matrix}$

where f_(v,exp) [dimensionless] is the particle volume fraction for theexperiment, X_(p) (g) is the weight of nanoparticles, ρ_(x) [g/m³] isthe density of nanocomposite powder and V_(y) [m³] is the volume ofbinder/matrix [23]. The volume fractions were identified forexperimental studies of nanoparticle-based radiative coolers.

Characterization of Thick Film Nanocomposites

The thick film nanocomposite was characterized using UV-Visspectroscopy, Fourier Transform Infrared (FTIR), Scanning ElectronMicroscope (SEM), Zeta sizer, and performance testing techniques.Samples were prepared on aluminum or a black reinforced polypropylene(PPR) geosynthetic substrates. The resulting spectral analysis providedinformation for tuning the radiative cooling of thick filmnanocomposite.

Infrared Spectroscopy

Since the primary atmospheric window is within the infrared spectrumthis research concentrated on infrared spectroscopy to obtain desiredspectral selectivity. Infrared spectroscopy was performed by JascoFTIR-6300 spectrometer and a Pike 30spec specular reflectance attachmentwith a variable aperture designed for the measurement of thick filmsheld the samples. To obtain a better understanding of FTIR graphsdisplay the logarithmic values of absorption (A) or (Abs) to tunetransmittance within the atmospheric window to the lowest levels. Thisstudy used the logarithmic Abs scale obtained with Equation 3 tomaximize Abs in the atmospheric window, which observed a peak Abs valueof 3 of means T<0.1% for the sample. Most published research normalizesthe absorptivity range from 0-1, however, an Abs value of 1 would equal10% T, so if you set the scale from 0-1, then you would only see thedata if it fell within the 10-100% T range [49].

A=2−log₁₀%T  (3)

A minimal amount of T over the spectrum was desirable to prevent thermalradiation from heating the substrate under thick film nanocomposite. Theabsorptivity results from FTIR spectral analysis are the function of alogarithmic equation, derived from Beer's Law, which convertstransmissivity (T) into absorptivity (A) using Equation 3.

Experiments which integrated spectrally selective nanoparticles into thethick film nanocomposites increased absorption levels over the entireinfrared spectrum, while the spectrum shape and absorption peaks closelyresembled that of the acrylic medium. Nevertheless, the more spectrallyselective the nanoparticle the more influence it will have in making thenanocomposite film more selective. For example, in FIG. 12 , since theRC nanoparticle's spectrum was more selective than the RC 9 and RC 16nanoparticles the RC 10's thick film had a higher level of reflectionoutside the atmospheric window to produce the spectrum closest to theideal spectral selectivity for passive radiative cooling. Theseexperiments demonstrated that it was possible to tune the thick filmnanocomposite's selectivity and absorption levels with nanoparticles ofdiffering spectral selectivity randomly dispersed in a medium. Spectralanalysis classified the cool roof coating as a broadband emitter due tothe strong absorption over the entire infrared spectrum, while the thickfilm nanocomposites containing selective nanoparticles were classifiedas selective emitters. A cool roof coating can be considered a thickfilm paste layer; however, it had a higher volume fraction of micro andnanosized particles and is on average ˜300 μm thicker than thick filmnanocomposite in the analysis.

Nanocomposite pastes including SiO particles and spectrally selectiveZnO nanoparticles, were formulated to maximize absorption in theatmospheric window while achieving some degree of selective emittance.The paste layer had an absorption peak of 99.9% within the atmosphericwindow; while the overall infrared emissivity between 6 to 10 μm wasgreater than 99.875%, where less than 0.125% of solar radiation istransmitted to substrate. Despite an absorption level over 99% and anabsorption peak in the atmospheric window the paste layer didn't exhibitsignificant cooling properties because the absorption outside the windowwas still too high. When a TiO₂ ink layer was placed over the paste,selectivity increased by increasing reflectance outside the atmosphericwindow on both sides. The TiO₂ layer slightly reduced the absorptionpeak in the window. As observed in the thermal imagery and measurementsby FLIR infrared radiometer/camera, the ink layer reduced surfacetemperature by an estimated 4° C. According to Wiley Spectrabase (2021)TiO₂ has its infrared absorption peak outside the atmospheric window ata wavelength of 20 μm or frequency of 500 cm-1 [14] which suggests amore spectrally selective nanoparticle ink layer on top of the pastecould increase selectivity and cooling to a greater degree. Experimentresults indicated the low-cost method of applying thin ink layers inthick film nanocomposites provide a feasible method for tuning andimproving passive radiative cooling.

UV-Vis Spectroscopy

The UV-Vis spectroscopy was performed with an Ocean Optics USB-2000UV-Vis-NIR spectrometer equipped with an enclosed steel chamber. Tuningfilms for high reflectance within 0.3-2.4 μm waveband are essential fordaytime radiative cooling due to the strong solar heat flux during theday [26].

The absorbance of the cool roof coating in the was significantly morethan the radiative cooling thick film nanocomposite. The strongultraviolet light absorbance of cool roof coatings was due to whitepigments such as titanium dioxide. Only 5% of the sun's radiation is inthe ultraviolet radiation region of the electromagnetic spectrum,however, this 5% is where the photons are energetic enough to excite atypical atom from the ground state to ionization [20].

Tuning doped ZnO nanoparticles for the lowest absorbance in UV-Visspectrum improves radiative cooling performance; however, the thicknessof the medium and amount of nanoparticles also factors into reducing UVabsorbance. The observed absorbance peak of ZnO was greatly reduced forPlate 4 and Plate 5 which had the same medium and volume fraction ofnanoparticles, but the coating was ˜20 μm thicker on Plate 5 whichresulted in less UV absorbance.

Scanning Electron Microscope (SEM)

The SEM analysis of thick film samples were performed by a Hitachi 5800or a Hitachi SU70 SEM. The thick film sample composition providesinsight into size, morphology, and spacing of particles.

The SEMs of nanocomposite film surfaces in FIGS. 18A-18D revealedrandomly distributed spectrally selective nanoparticles. Particlesspacing of 5-10 μm apart with slight agglomeration is observed in FIGS.18B-18C. Agglomeration occurred in coatings with randomly dispersedparticles in a medium and applied by cost effective means. Because ofthis larger range of particle sizes and agglomeration absorption andreflectance was enhanced throughout the spectrum also in part because ofthe spectral selectivity of the materials. Nevertheless, both filmsexhibited spectrally selective properties, because of the spectralselectivity of the binder and nanoparticles.

The paste absorption reached over 99% in the atmospheric window, yet itdidn't exhibit cooling properties. The SiO alone did not obtainradiative cooling but can be used as a filler if higher absorption isrequired.

Discussion

The thick film nanocomposites fabricated in this research were subjectedto multiple performance measurement methods. All testing involvedexposure of samples to direct sunlight. Direct sunlight can beclassified as sunlight without filters between the object and sunlight.In other words, they were not placed inside an insulated chamber with aconvection barrier. Also, the temperature readings were of the samplesthemselves and not of an enclosed air space below.

A side-by-side comparison of technologies is an insightful way topresent any type of performance data. Exposure of the thick filmnanocomposite coatings and cool roof coating on same aluminum substrateto the same environmental conditions proved to be a good method to gaugeprogress throughout this research. Disproportionate performancereporting resulting from nonstandard testing procedures is eliminatedsince both materials are tested under exact conditions.

Thermal images and photographs were obtained using a FLIR “forwardlooking infrared radiometer” E5 2.0L infrared camera. This method oftesting provides a good option to compare and visualize infraredemittance from samples. Each pixel in image represented a surfacetemperature so one can assess the whole surface not just one point whichis a limitation of thermocouple measurements. In FIG. 12 the scale tothe right of image has white as the hottest temperature while purple isthe coldest. A spectrally selective ink layer on aluminum substrate indirect sunlight exhibited greater cooling power than cool roof coating.

Ambient conditions for testing were 29° C., 78% humidity, and wind of 8mph. The side by-side comparison of thick film nanocomposites to a coolroof coating on aluminum substrates in FIG. 12 , provide a positiveindication that the thick film's cooling power can be significantlyhigher than a cool roof coating. The samples on aluminum plates are acool roof coating in (a)—a 440 μm thick paste layer. Nanoparticle andsiloxane mixture sprayed on aluminum substrate for Plate 11 in (c)—47 μmthick w/5% particle #37 and Plate 21 in (b)—40 μm thick w/5% particle#39. While a chemical solution deposition of siloxane mixture for Plate24 in (d)—58 μm thick w/5% particle #10. Application of nanoparticlesadhered to acrylic binder surface for Plate 33 in (e) ˜30 μm thick w/5%particle #44. Note that the lighter color on the bottom on (c) wasfeedback from the thermal camera. Nevertheless, the cool roof coatingwas above ambient temperature while the thick film nanocomposite inklayers whether in siloxane or acrylic paint were still below ambient.Also, this experiment demonstrated the ability to tune the emissivity ofthe thick film nanocomposite by varying medium and spectrally selectivenanoparticles.

Infrared thermometers quickly measure the temperature for a point on anexposed material's surface. Thermal assessments of prepared samples wereperformed by EXTECH Instruments, Model 42515, Wide Range InfraRed (IR)Thermometer with a laser pointer. The highest average monthly solarradiation for the Florida testing location at Latitude 27.85° N,Longitude 82.78° W is 7.31 kWh/m²/day for May, while the lowest inDecember is less than half with 3.34 kWh/m²/day. Furthermore, the solarradiation intensity is highest during solar noon when the angle ofincidence is 90 degrees. Measurements were also taken during the time ofyear and time of day when the highest solar radiation is incident upon asample's surface. The surface temperature of the thick filmnanocomposite (left) was 108.3° F. which is over 17° F. cooler than thecool roof coating (right) with a surface temperature of 125.4° F. Arepeat of this experiment in December would record much lower surfacetemperatures.

Despite not posting surface temperatures below ambient temperatures thethick film nanocomposite and the cool roof coating still providedpassive cooling to a greater degree than non-coated materials orstandard roofing materials. ˜60° F. hotter during day. On a day reaching91° F. heat-absorbing materials like conventional roofing materials heatup ˜150° F. [2]. Still both below 60° F. temperature increase ofstandard roofing materials Concentrating on daytime radiative cooling isimportant since many materials in city absorb a lot of radiation duringthe day, then retain this heat for a long time.

Thermal assessments of prepared samples were performed by an EXTECHInstruments, EasyView 11A, Type K Thermometer with a Type Kthermocouples. An exposed sample with a Type K thermocouple affixed tothe backside by Kapton tape consistently recorded temperatures belowambient during the majority of a 24-hour period; however, when testedduring times when the greatest solar radiation was incident upon itssurface the temperatures were above ambient yet not as high as infraredthermometer measurements. Once the angle of incidence upon the samplesdecreased the temperatures declined below ambient temperatures.

Despite testing during the highest solar radiation of the year theexposed plate temperatures during the latter part of the day in FIG. 17were ˜4-11° F. below ambient temperatures. This cooling is attributed tothe spectral selectivity of the thick film nanocomposite [67].

The methods and compositions of the appended claims are not limited inscope by the specific methods and compositions described herein, whichare intended as illustrations of a few aspects of the claims and anymethods and compositions that are functionally equivalent are within thescope of this disclosure. Various modifications of the methods andcompositions in addition to those shown and described herein areintended to fall within the scope of the appended claims. Further, whileonly certain representative methods, compositions, and aspects of thesemethods and compositions are specifically described, other methods andcompositions and combinations of various features of the methods andcompositions are intended to fall within the scope of the appendedclaims, even if not specifically recited. Thus, a combination of steps,elements, components, or constituents can be explicitly mentionedherein; however, all other combinations of steps, elements, components,and constituents are included, even though not explicitly stated.

The following publications as listed below and throughout this documentare hereby incorporated by reference in their entirety herein.

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1.-34. (canceled)
 35. A method for forming spectrally selectivenanoparticles, the method comprising: heating a growth solutioncomprising a zinc salt precursor comprising one or more zinc salts, zincoxide seed particles, and one or more dopants in a non-pressurizedhydrothermal reactor to a first temperature under agitative conditionsfor a reaction period; cooling the reactor to a second temperature lessthan the first temperature for a cooling period to form a precipitate ofrecrystallized doped zinc oxide nanoparticles dispersed in a suspension;and separating and collecting the recrystallized nanoparticles from thesuspension, wherein the collected nanoparticles exhibit a spectralselectivity in the atmospheric window.
 36. The method of claim 35,wherein the one or more zinc salts comprise zinc acetate, zinc nitratehexahydrate, or a combination thereof.
 37. The method of claim 35,wherein the one or more dopants is selected from the group consisting ofAg₂O, Al₂O₃, CaCO₃, Cu₂O, MgO, SiC, SiO, SiO₂, SiO₄, Si₃N₄, SnO₂, TiO₂,Fe₃O₄, VO₂, SnO, CeO, and combinations thereof.
 38. The method of claim35, wherein the growth solution comprises two or more dopants andwherein at least one of the two or more dopants comprises a noble metal.39. The method of claim 35, wherein the growth solution comprises two ormore dopants and wherein the two or more dopants comprise Si₃N₄ andSiO₂.
 40. The method of claim 35, wherein the zinc oxide seed particlescomprise doped zinc oxide nanoparticles.
 41. The method of claim 35,wherein the nanoparticles are substantially polydisperse, substantiallypolymorphic, and/or substantially polycrystalline.
 42. The method ofclaim 35, wherein the step of separating and collecting the doped zincoxide nanoparticles comprises decanting the solution, washing theprecipitated doped zinc oxide nanoparticles at least once, andsubstantially drying the doped zinc oxide nanoparticles.
 43. The methodof claim 35, wherein the zinc oxide seed particles are continuallysuspended in the growth solution for substantially the entire reactionperiod.
 44. A composition comprising: a population of polycrystallinezinc oxide nanoparticles doped with one or more dopants, wherein thepopulation of nanoparticles is spectrally selective in the atmosphericwindow.
 45. The composition of claim 44, wherein the one or more dopantsis selected from the group consisting of Ag₂O, Al₂O₃, CaCO₃, Cu₂O, MgO,SiC, SiO, SiO₂, SiO₄, Si₃N₄, SnO₂, TiO₂, Fe₃O₄, VO₂, SnO, CeO, andcombinations thereof.
 46. The composition of claim 44, wherein the oneor more dopants includes two or more dopants, and wherein at least oneof the two or more dopants comprises a noble metal.
 47. The compositionof claim 44, wherein the one or more dopants comprise Si₃N₄ and SiO₂.48. A thermal control coating (TCC) comprising the population ofpolycrystalline zinc oxide nanoparticles according to claim
 44. 49. Ananocomposite comprising: a substrate comprising one or more layersdeposited thereon, wherein at least one of the one or more layerscomprises a radiative cooling layer comprising doped zinc oxidenanoparticles spectrally selective in the atmospheric window.
 50. Thenanocomposite of claim 49, wherein the substrate comprises aluminum,silver, an alloy thereof, or a combination thereof.
 51. Thenanocomposite of claim 49, wherein the doped zinc oxide nanoparticlescomprise one or more dopants selected from the group consisting of Ag₂O,Al₂O₃, CaCO₃, Cu₂O, MgO, SiC, SiO, SiO₂, SiO₄, Si₃N₄, SnO₂, TiO₂, Fe₃O₄,VO₂, SnO, CeO, and combinations thereof.
 52. The nanocomposite of claim49, wherein the nanoparticles are substantially polydisperse,substantially polymorphic, and/or substantially polycrystalline.
 53. Thenanocomposite of claim 49, wherein the radiative cooling layer has athickness of from 40-300 μm.
 54. The nanocomposite of claim 49, whereinthe radiative cooling layer comprises a filler having the doped zincoxide nanoparticles randomly distributed therein.